3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a wireless device such as a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as an evolved NodeB (eNodeB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. The eNodeB is a logical node in LTE and the RBS is a typical example of a physical implementation of an eNodeB.
FIG. 1 illustrates a radio access network in an LTE system. An eNodeB 101a serves a UE 103 located within the eNodeB's geographical area of service also called a cell 105a. The eNodeB 101a is directly connected to the core network. The eNodeB 101a is also connected via an X2 interface to a neighboring eNodeB 101b serving another cell 105b. Although the eNodeBs of this example network serves one cell each, an eNodeB may serve more than one cell.
Robust and fast HandOver (HO) is one of the key technologies in an LTE system. An LTE HO is basically a hard HO, which means that there will be a user plane interruption when the UE is handed over from one serving cell to another. A fast HO is therefore beneficial for the data delivery. FIG. 2 is a signaling diagram schematically illustrating a HO procedure in the LTE system. The LTE HO procedure is initiated by the measurement report message 210 sent from the UE 103 to the serving eNodeB 101a. The measurement report triggers the HO of the UE, and is based on periodical channel quality measurements of the neighboring cells performed by the UE. The channel quality measurements are either Reference Signal Received Power (RSRP) measurements or Reference Signal Received Quality (RSRQ) measurements. When one of the measured neighboring cells has an RSRP or RSRQ which is higher than the RSRP or RSRQ of the cell serving the UE for a given time period, that neighboring cell is a candidate target cell for the HO of the UE, and the UE will report the cell and the corresponding measurements in the measurement report. The RSRP/RSRQ of the neighboring cell must exceed the RSRP/RSRQ of the serving cell with a certain amount of dB, corresponding to the hysteresis threshold, under the given time period, referred to as the time to trigger threshold. The measurement report may comprise measurements for more than one neighboring cell, sorted in the order of the channel quality.
The serving eNodeB 101a subsequently makes a HO decision 211 based on the measurement report. The eNodeB 101a selects one of the reported neighboring cells as the new serving cell also referred to as the target cell, and sends a HO request message 212, including the user context information to the target eNodeB 101b controlling the target cell over the X2 interface. If the target cell is not overloaded, the target eNodeB 101b will reserve resources in 213, and respond to the serving eNodeB 101a with a HO request acknowledgement (ACK) message 214 comprising a dedicated Random Access (RA) preamble to be used by the UE when accessing the target cell.
The time it takes from the sending of the X2 message referred to as the HO request 212, to the reception of the X2 message referred to as the HO request ACK 214, may be between 10 ms and 50 ms, depending on whether the HO is intra eNodeB or inter eNodeB. In some cases, if the first selected target cell is congested and the first target eNodeB thus replies to the HO request with a non-acknowledgement, the serving eNodeB 101a may have to resend the HO request 212 to another target eNodeB controlling another non-congested target cell. The total delay for the four X2 messages in this case may then be more than 100 ms.
Upon reception of the HO request ACK message 214, the serving eNodeB 101a will send a HO command 215 comprising the dedicated RA preamble to the UE 103. Thereafter, the UE 103 disconnects with the serving cell, and tries to access the target cell by sending a RA message 216 to the target eNodeB 101b using the RA preamble and by setting up the RRC connection.
The delay for the X2 HO signaling messages may thus take up to 100 ms when the selected target cell has no available resources for the new coming access. The reason for such long delay is that the serving eNodeB will have to try a second neighbor cell before it can secure resources for the UE. If also the second cell is congested, a third access may have to be performed. These extra delays will negatively impact the overall HO performance.
FIG. 3 is a schematic illustration of a possible effect of an X2 delay. Because of the X2 delay in the HO procedure, the HO command message 215 is inevitably delayed. The UE 103 may thus be moving a certain distance 301 during the delay between the transmission of the measurement report triggering the HO and the reception of the HO command. This means that the UE may be close to the serving cell border when it is time for the serving eNodeB 101a to transmit the HO command. At the cell border the channel quality may be bad and may not support a reliable transfer of the HO command. Hence, this will increase the possibility of a HO failure due to a low quality HO command transmission. In heterogeneous network scenarios, HO events may be triggered much more frequently, and the above described problem may become even more severe.
Furthermore, it has been observed that HO performance would be degraded significantly when a Discontinuous Reception (DRX) feature is applied. In such a case, the UE cannot continuously measure the downlink channel quality, and the HO delay is much higher than the typical values. It is therefore believed that the above described problem will be even more severe when using DRX.